1. Field of Invention
The invention relates to electronic flash devices having particular utility with low cost photographic cameras, and more specifically to charging and charge-control circuits for such flash devices.
2. Description of the Prior Art
The above-cited cross-referenced application discloses an electronic flash device usable in a camera. The device includes a self-oscillating charging circuit, a momentary trigger for initiating oscillations of the circuit to charge a flash capacitor, and a voltage sensing device coupled directly between the circuit and the flash capacitor for arresting the oscillations when the capacitor is fully charged. The voltage sensor includes a zener diode in series with a neon ready-light, and switches on a transistor to ground the charging circuit and arrest the oscillations when the capacitor is fully charged. Actuation of the flash device automatically restarts the oscillations and recharges the flash capacitor.
In the electronic flash device, a transistor 46 is forward biased when a switch 42 is momentarily closed, thereby starting the charger. However, a small pulse of static electricity at the base of transistor 46 will also start the charger because transistors 46 and 48 are connected in a manner that provides extremely high current gain. Thus, a problem with the electronic flash device is that an external static electric discharge to the camera can start the oscillations in the charging circuit, thereby charging the flash capacitor. For example, bringing a static charged object, such as a piece of clothing, near the camera will start the charging cycle, just as if the momentary trigger had been actuated. Such a result is undesirable because it reduces battery life.